Spanish duo Albert Ramos-Vinolas and Fernando Verdasco suffered defeat in their respective first-round matches at the St Petersburg Open on Tuesday.
Sixth-seed Ramos Vinolas lost 7-5 6-2 to world number 255 Alexandre Sidorenko in a match that lasted one hour and 23 minutes.
The victory was Sidorenko's first on the ATP Tour in 2016, and he was pleased it came in his home town.
"I'm really happy I won my first main draw match here. It's always nice to win in your native city in front of your home fans, friends and family," he said in a post-match interview.
Next up for Sidorenko is Paolo Lorenzi, who staved off two match points to overcame Mischa Zverev 3-6 7-6 (8-6) 6-0.
Former world number seven Verdasco also fell at the first hurdle, despite taking a one-set lead against Dusan Lajovic.
Serbian Lajovic dominated the Spaniard's second serve as he rallied from behind to win 2-6 6-3 6-2 and book a place in the second round.
In an all-Russian clash, qualifier Daniil Medvedev eased past wildcard Alexander Bublik 6-4 6-0.